Lithium-ion secondary batteries are well known as examples of nonaqueous electrolyte secondary batteries. Lithium-ion secondary batteries are popular in smart phones, tablets, notebook computers and other mobile devices because they have superior energy density, output density, charge-discharge cycle characteristics and the like in comparison with other secondary batteries such as lead storage batteries, and they have contributed to reducing the size and weight and increasing the performance of such devices. However, in terms of output, time required for charging and the like, they have not yet reached the level of performance required of secondary batteries for use in electrical vehicles and hybrid vehicles (vehicle-mounted secondary batteries). Therefore, research is being conducted to improve the charge-discharge characteristics at high current densities (high-rate characteristics) with the aim of increasing the output and reducing the charging times of nonaqueous electrolyte secondary batteries. Also, since high durability is also required for vehicle-mounted applications, compatibility with cycle characteristics is required.
Nonaqueous electrolyte secondary batteries are composed of a pair of electrodes disposed with a separator in between and a nonaqueous electrolyte solution. Each electrode is formed of a collector and a mixture layer formed on the surface of the collector, and the mixture layer is formed by, for instance, coating and drying an electrode mixture layer composition (slurry) containing an active material, a binder and the like on the collector.
Meanwhile, in recent years, aqueous electrode mixture layer compositions have also been in increased demand for reasons such as environmental protection and cost reduction. In the context of lithium-ion secondary batteries, aqueous binders using styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are being used in electrode mixture layer compositions for negative electrodes that use carbon materials such as graphite as the active material. However, further improvements are needed to accommodate the advanced high-rate characteristics and cycle characteristics required for vehicle-mounted applications. Meanwhile, solvent-based binders of polyvinylidene fluoride (PVDF) and the like using organic solvents such as N-methyl-2-pyrrolidone (NMP) are preferred for the positive electrodes of lithium-ion secondary batteries, and no aqueous binder has been proposed that fulfills the requirements discussed above.
Carbon-based materials including active materials such as graphite and hard carbon (HC) and conductive aids such as Ketjen black (KC) and acetylene black (AB) are widely used as components of lithium-ion secondary batteries. These carbon-based materials generally have poor wettability to aqueous media, so to obtain a uniform electrode mixture layer composition with excellent dispersion stability, an aqueous binder having an excellent dispersion stabilizing effect on these carbon-based materials is desired. As shown below, an aqueous binder containing a cross inked polyacrylic acid has been proposed as an aqueous binder that is applicable to a lithium-ion secondary battery electrode.
Patent Literature 1 discloses an acrylic acid polymer crosslinked with a polyalkenyl ether as a binder for forming a negative electrode coating of a lithium-ion secondary battery. Patent Literature 2 describes how an excellent capacity retention rate can be obtained without destruction of the electrode structure even when using an active material containing silicon by using as a binder a polymer comprising a polyacrylic acid crosslinked with a specific crosslinking agent.
Crosslinked polyacrylic acids are also widely used as water-soluble thickeners, and Patent Literature 3 discloses an unsaturated carboxylic acid-based crosslinked polymer obtained by including a specific crosslinking agent.
Patent Literature 1 Japanese Patent Application Publication No. 2000-294247
Patent Literature 2 International Publication No. 2014-065407
Patent Literature 3 Japanese Patent Application Publication No. 2010-132723